Excise applicability on good supplied and installed

Dear Sir,

I am in to a business of design, supply, installation of systems. None of the items required for any

system is manufactured by us. The orders received by us are for supply of different components of the system and I&C at customer's site. The billing done is VAT or CST for supply and Service Tax for I&C. If the trunover cross the excise limit of 150 Lacs.

My questions are as follows.

a) Does my busines come under excisable after crossing the limit of 150 Lac.
b) If yes then from when I have to charge and pay excise? On the invoice on which I am crossing the excise limit.
c) Limit of 150 Lac is inclusive of Turnover with invoice value with tax or without tax.
d) Do I have to consider the invoices with service taxes (Like AMC or repairing charges) also in total turnover
for limit.